Variable magnetic electron lens



March 31, 1959 A. c. VAN DORSTEN 2,880,324

VARIABLE MAGNETIC ELECTRON LENS Filed Sept. 11, 1953 7 4 INVENTOR.

ADRZANNUS 6612mm Z NDozas TEN A (IL I United States Patent "ice VARIABLEMAGNETIC ELECTRON LENS Adrianus Cornelis van Dorsten, Eindhoven,Netherlands,

Application September 11, 1953, Serial No. 379,716

Claims priority, application Netherlands September 16, 1952 8 Claims.(Cl. 250-495) The invention relates to magnetic electron lenses for usein electron microscopes and similar apparatus It provides means ofvarying in a simple manner the magnetic fieldof such a lens and ofcorrecting-certain defects of the lens.

, According to the invention a movable, cylindrical, magnetic insert isarranged axially in the bore of a pole piece for use in a magneticelectron lens. It has been found that varying the position of such aninsert enables the properties of the lens to be varied. Thus, axialshifting of theinsert permits of varying the focal length of the lens.In my copending application, Serial No. 578,990, filed June 16, 1954,the effect of an insert in thecase of magnetic saturation'is described,if spaceis available between the insert and the bore wall. In the polepiece according to the invention there is no need for said space to beavailable, it being possible for the focal length to be varied by meansof a sliding insert which fits into the pole piece bore.

Such an exactly fitting insert does not provide the magnetic screeningeffect of a disengaged tube, since it is saturated together with thesurrounding material.

The movability of the magnetic insert may be useful in view of somedefects occurring in the magnetic electron lens. It is known, forexample, that there is a relationship between the form of the fieldcurve (the curve showing the magnetic field strength in the lens axis asafunction of the distance along the axis) and the spherical aberration.Thus, adjustment of the axially movable inserts (which may be difierentfor either pole piece) enables the spherical aberration to be corrected.

In addition, the movable insert permits of compensating for orcorrecting the astigmatism of the lens, which is a departure fromrotation symmetry.

The invention will now be described with reference to the accompanyingdiagrammatic drawing, in which:

Fig. 1 is a cross-sectional view of a pole piece according to theinvention taken along a plane passing through the axis, and

Figs. 2, 3 and 4 show separate insert cylinders which may be used toprovide correction for astigmatism.

Referring now to Fig. l, pole pieces 1 and 7 of a magnetic electron lensfor use in an electron microscope or an electron diffraction apparatusare shown. The front face 2 of the pole pieces 1 and 8 of pole piece 7constitute the pole faces. They are separated by a spacer 9 ofnon-magnetic material. A copper tube 3 passes through the bore of thepole pieces. It acts to exclude the ambient atmosphere from a spacearound the axis, which is done by the tube being evacuated so as toallow the electrons to travel through it.

The pole pieces 1 and 7 are connected through a yoke 4 extending betweenthem and which surrounds an energizing winding 10 which produces amagnetic flux in the yoke.

A cylindrical insert movably arranged according to the inventionsurrounds the tube 3. The cylindrical 2,880,324 Patented Mar. 31, 195?insert is extended at the end more remote from the pole face 2 andprovided with external screw thread by means of which it fits into thewider part of the pole piece 1.

An enlarged portion 6 projecting beyond the pole piece enables theinsert to be actuated so as to screw it outwardly or inwardly and thusto vary the magnetic field at the pole face adjacent the axis.

The use of an insert 5 which, although symmetrical with respect to twoplanes passing through the axis at right angles to each other, i.e.,orthogonal planes ha'ving the lens axis on their line of intersection,is not a true solid of revolution allows variation of the astigmatism ofthe lens. The use of an insert shaped into such a form has the effect ofintroducing an artificial astigmatism which together with the naturalastigmatism of the lens produces a resultant astigmatism. Rotation ofthe insert about its axis has the effect of varying the resultantastigmatism, which exhibits a minimum value in a particular position ofthe insert. Axial displacement increases or decreases the effect of theinsert and consequently permits variation of the degree of correction.It the defect for which correction is required is of thesame order ofmagnitude as the artificial one,'it can be compensated for in thismanner. I

A cylindrical insert 5 by which this correction effect is ensured isshown in Fig. 2 viewed in the axial direction. The outer wall of thiscylinder is truly circular but the inner wall thereof is elliptical.However, the centre of the elliptical inner cross-section coincides withthe centre of the circular outer cross-section and this ensures symmetrywith' respact to two planes passing through the axis at right angles toeach other. Thus, the wall has a varying thickness which is at itsminimum in one plane of symmetry and at its maximum in the other.Rotation of this cylinder through enables the resultant astigmatism toassume any desired value.

As an alternative, the edge of the end of the cylindrical insert nearerthe pole face may be given a height varying along the circumference.This will generally work out as a length of the cylindrical insertvarying along the circumference. Also in this case there must besymmetry with respect to two planes passing through the axis at rightangles to each other. Embodiments of a cylindrical insert 5 formed intosuch a shape are shown in Figs. 3 and 4. The profile may be toothed asshown in Fig. 3, its developed surface being bounded by a broken line.It may alternatively be a curve or exhibit a form as shown in Fig. 4,the developed surface of which exhibits a curve, for example, asinusoidal line. The elevations and depressions are identical in formand two cylinders as shown in Fig. 3 or 4 have interlocking profilededges. This assists in an even variation of the astigmatism andconsequently in easy adjustment.

The astigmatism of the lens may generally be looked upon as thecombination of a fundamental departure from normal and one or moredepartures of higher order. To correct the latter, provision may be madeof separately adjustable cylinders co-axial with the others. Since eachof these components has a sinusoidal variation, this form is mostsuitable for the profile of the edge.

What is claimed is:

1. A magnetic electron lens comprising a pair of spaced apartferromagnetic pole shoes defining a gap therebetween, said pole shoeshaving axially-aligned bores extending therethrough and communicatingwith said gap, and a hollow, ferromagnetic insert member rotatablymounted within the bore of one of the pole shoes and axially movabletherewithin, said ferromagnetic insert having an end portion,communicating with said gap, that has a center of symmetry and is alsosymmetrical with respect .to orthogonal planes having the and having acenter of symmetry at the bore axis and .having mirror symmetry withrespect to each of orthogonal planes having the bore axis on their lineof .inter .section, whereby rotation ofsaid insert member changes thefield distribution in the gap and enables the lens astigmatism to becorrected.

3. A lens as set forth in claim 2 wherein said .ferromagnetic insert isalso axially displaceable Within said bore.

4. A magnetic electron lens comprising apair of spaced apartferromagnetic pole shoes defining a gap ttherebe- "tween, said poleshoes having axially-aligned bores extending therethrough andcommunicating with said gap, and a soft ferromagnetic substantiallycylindrical hollow insert member rotatably mounted within the bore ofone of the pole shoes, said insert having a.hollow end portion,communicating with said gap, whose outer and inner periphery have theshape of a circle and ellipse,

respectively, having a common center, said insert also .having mirrorsymmetry with respect'to each of orthogonal planes having the bore axisas their-line of intersection, whereby rotation of said insert memberenables astigmatism of the lens to be corrected.

5. A magnetic electron lens comprising a pair of spaced apartferromagnetic pole shoes defining a gap therebetween, said pole shoeshaving axially-aligned bores extending therethrough and communicatingwith said gap, and a ferromagnetic substantially cylindrical hollowinsert member rotatably mounted within the bore of one of the poleshoes, said insert having a hollow end portion, communicating with saidgap, with axially-projecting portions of different length, said endportion also having mirror symmetry with respect to each of orthogonalplanes having the bore axis as their line of intersection, wherebyrotation of said insert enables astigmatism of the lens to be corrected.

6. A lens as set forth in claim 5 wherein said end portion of saidinsert member has a toothed profile.

7. A lens as set forth in claim 5 wherein said end portion of saidinsert member is shaped in the form of a sinusoidal line.

8. A lens as set .forth .in claim 5 wherein said insert member is alsoaxially displaceable along the bore,

References Cited in the file of this patent UNITED STATES PATENTS

